The prospect of the UK crashing out of the European Union in October is now a "very serious" risk, a senior Irish foreign ministry official said.

"Since March, there is an exhaustion with Brexit," Rory Montgomery, who has helped oversee Ireland's preparations for the UK's exit from the bloc, said at an event in Nicosia on Wednesday. "The British government need to ask for an extension and it's going to be difficult for a Conservative leader to do so."

Boris Johnson, front-runner to replace Theresa May as prime minister, said on Wednesday he doesn't want a no-deal Brexit but the UK must prepare for one as a negotiating tactic. Like most other contenders for the job, he is promising to demand concessions from the bloc on the backstop, the device designed to avoid the re-emergence of a hard border in Ireland.

Dialing down his earlier rhetoric, Johnson said he wants a "sensible, orderly" divorce, but stuck with his vow to leave the bloc on October 31.

Montgomery said he sees no sign that EU support for Ireland will fracture.

"There is a very strong sense that Ireland is a member state and the UK is leaving so I don't think they will throw us under the bus," he said.